At the beginning, a popular method of deoxidizing steel utilized ferrotitanium for preparing steel deoxidized with Ti, for example, as disclosed in Japanese Patent Publication (JP-B) Sho-44-18066. Recently, however, a large amount of steel has been deoxidized with Al and has an Al content of not smaller than 0.005% by weight. This is done in order to obtain steel having a stable oxygen concentration at low production cost.
For producing steel deoxidized with Al, vapor stirring or RH-type vacuum degassing is employed, in which the oxide formed is coagulated, floated on the surface of steel melt and removed from the steel melt. In that method, however, the formed oxide Al.sub.2 O.sub.3 inevitably remains in the steel slabs. In addition, the oxide Al.sub.2 O.sub.3 is formed in clusters and is therefore difficult to remove. As the case may be, cluster-type oxide inclusions of not smaller than hundreds of .mu.m in size may remain in the deoxidized steel. Such cluster-type inclusions, if trapped in the surfaces of the slabs, will produce surface defects such as scabs or slivers, which are fatal to steel sheets for vehicles that are required to have good exterior appearance. In addition, the Al deoxidation method is further disadvantageous in that formed Al.sub.2 O.sub.3 will adhere onto the inner wall of the immersion nozzle for steel melt injection from the tundish to the mold, thereby causing nozzle clogging.
For overcoming the problems of the Al deoxidation method, a proposed method added Ca to the aluminium-killed steel melt to form composite oxides of CaO/Al.sub.2 O.sub.3. (For example, see Japanese Patent Application Laid-Open (JP-A) Sho-61-276756, Sho-58-154447 and Hei-6-49523).
The object of Ca addition was to react Al.sub.2 O.sub.3 with Ca thereby forming low-melting-point composite oxides such as CaOAl.sub.2 O.sub.3, 12CaOAl.sub.2 O.sub.3, 3CaOAl.sub.2 O.sub.3 and the like to overcome the problems noted above.
However, adding Ca to steel melt results in formation of CaS through reaction of Ca with S in the steel, and the resulting CaS causes rusting. In this respect, JP-A Hei-6-559 has proposed a method of limiting the amount of Ca allowed to remain in steel to from 5 to less than 10 ppm for the purpose of preventing rusting. However, even if the Ca amount is so limited to less than 10 ppm, when the composition of the CaO--Al.sub.2 O.sub.3 oxides remaining in the steel is not proper, especially when the CaO content of the oxides is not smaller than 30%, then the solubility of S in the oxides increases whereby CaS is inevitably formed around the inclusions while the steel melt is being cooled or solidified. As a result, the steel sheets tend to rust from the starting points of CaS, and have poor surface properties. If the steel sheets thus having rusting points are directly surface-treated for galvanization or coating, the surface-treated sheets do not have a uniform good surface quality.
On the other hand, if the CaO content of the inclusions is not larger than 20% but the Al.sub.2 O.sub.3 content is high, especially when the Al.sub.2 O.sub.3 content thereof is not smaller than 70%, the inclusions shall have an elevated melting point and will be easily sintered together, thereby creating still other problems; nozzle clogging is inevitable during continuous casting, and, in addition, many scabs and slivers are formed on the surfaces of steel sheets to the detriment of surface properties.
A steel deoxidation method using Ti but not Al has been disclosed in JP-A Hei-8-239731. No cluster-type oxides are formed, but the ultimate oxygen concentration in the deoxidized steel is high and there are numerous inclusions as compared with the Al deoxidation method. In particular, in the Ti deoxidation method, the inclusions formed are in the form of Ti oxides/Al.sub.2 O.sub.3 composites which are in granular dispersion of particles of from about 2 to 50 .mu.m in size. Accordingly, in that method, the surface defects caused by cluster-type inclusions are reduced. However, the Ti deoxidation method remains disadvantageous in that, for steel melt with Al.ltoreq.0.005% by weight, when the Ti concentration in the melt is 0.010% by weight or more, the solid-phase Ti oxides formed adhere to the inner surface of the tundish nozzle while carrying steel therein, and continue to grow, thereby inducing nozzle clogging.
In order to solve the nozzle clogging problem, JP-A Hei-8-281391 has proposed a modification of the Ti deoxidation method not using Al, in which the oxygen content of the steel melt that passes through the nozzle is controlled, in order to prevent growth of Ti.sub.2 O.sub.3 on the inner surface of the nozzle. However, since the oxygen control is limited, the method is still disadvantageous in that the castable amount of steel is limited (up to 800 tons or so). In addition, with the increase of nozzle clogging the level control for the steel melt in the mold becomes unstable. Thus, in fact, the proposed modification cannot provide any workable solution of the problem.
According to the technique disclosed in JP-A Hei-8-281391, which is designed to prevent tundish nozzle clogging, the Si content of the steel melt is optimized to form inclusions having a controlled composition of Ti.sub.3 O.sub.5 --SiO.sub.2 whereby the growth of Ti.sub.2 O.sub.3 on the inner surface of the nozzle is prevented. However, the mere increase of Si content could not always result in the intended formation of SiO.sub.2 in the inclusions, for which at least the requirement of (wt. % Si)/(wt. % Ti)&gt;50 must be satisfied. Accordingly, in the proposed method, where the Ti content of steel to be cast is 0.010% by weight, the Si content thereof must be not smaller than 0.5% by weight in order to form SiO.sub.2 --Ti oxides. However, the increase in the Si content hardens the steel material while worsening the galvanizability of the material. Specifically, the increase in the Si content has significant negative influences on the surface properties of steel sheets. Accordingly, the proposal in JP-A Hei-8-281391 still cannot produce any radical solution of the problem.
JP-B Hei-7-47764 has proposed a non-aging, cold-rolled steel sheet that contains low-melting-point inclusions of 17 to 31 wt. % MnO--Ti oxides, for which steel is deoxidized to an Mn content of from 0.03 to 1.5% by weight and a Ti content of from 0.02 to 1.5% by weight. In this proposal, the MnO--Ti oxides formed have a low melting point and are in a liquid phase in the steel melt. The steel melt does not adhere to the tundish nozzle while it passes therethrough, and is well injected into a mold. Thus, the proposal is effective for preventing tundish nozzle clogging. However, as so reported by Yasuyuki Morioka, Kazuki Morita, et al. in "Iron and Steel", 81 (1995), page 40, the concentration ratio of Mn to Ti in steel melt must be (wt. % Mn)/(wt. % Ti)&gt;100 in order to form the intended MnO--Ti oxides having an MnO content of from 17 to 31%. This is because of the difference of oxygen affinity between Mn and Ti. Therefore, when the Ti content of steel to be cast is 0.010% by weight, the Mn content thereof must be at least 1.0% by weight in order to form the intended MnO--Ti oxides. However, too much Mn, more than 1.0% by weight in steel, hardens the steel material. For these reasons, therefore, it is in fact difficult to form the inclusions of 17 to 31 wt. % MnO--Ti oxides.
JP-A Hei-8-281394 has proposed another modification for preventing tundish nozzle clogging in the method of Al-less deoxidation of steel using Ti, in which a nozzle is used that is made from a material that contains particles of CaO/ZrO.sub.2. In the proposed modification, even when Ti.sub.3 O.sub.5 formed in the steel melt is trapped in the nozzle, it is converted into low-melting-point inclusions of TiO.sub.2 --SiO.sub.2 --Al.sub.2 O.sub.3 --CaO--ZrO.sub.2 and is prevented from growing further.
In that modification, however, when the oxygen concentration in the steel melt being cast is high, the TiO.sub.2 content of the adhered inclusions shall be high so that the inclusions could not be converted into the intended low-melting-point ones. In that case, the proposed modification cannot produce the intended result of preventing nozzle clogging. On the other hand, when the oxygen concentration in the steel melt is low, another problem arises: the nozzle is fused and damaged. In any event, the proposed modification is not a satisfactory measure for preventing nozzle clogging.
The prior art techniques noted above for preventing nozzle clogging, when applied to continuous casting, still require blowing of Ar gas or N.sub.2 gas into the immersion nozzle through which the steel melt being cast is injected through the tundish nozzle into the mold. However, this is still disadvantageous in that the gas blown into the immersion nozzle tends to be trapped in the coagulation shell to form blowhole defects.